1. Field of the Invention
The present invention relates to a micro magnetic field sensing device and a fabrication method of the same.
2. Description of the Related Art
A magnetic sensor composed of a magnetic substance and coils is highly sensitive to very small applied magnetic fields. In general, a magnetic sensor is fabricated by winding coils around a soft magnetic core, and requires an electronic circuit for generating a magnetic field proportional to a magnetic field measurement. A magnetic field sensing device for the magnetic sensor can be realized by a thin-film soft magnetic core and planar thin film coils.
FIGS. 1A to 1H are diagrams for explaining a related art magnetic field sensing element integrated with a substrate, and a fabrication method of the same. Referring to the drawings, the structure of the magnetic field sensing device and its fabrication method will now be described.
As shown in FIG. 1H, the magnetic field sensing device includes a substrate 1, a magnetic substance 2, a first and a second insulating film 3 and 4, and a first and a second coil 5, 6 formed of a plurality of wires for encompassing the magnetic substance 2. Here, the first coil 5 is disposed at the lower portion of the magnetic substance, whereas the second coil 6 is disposed at the upper portion of the magnetic substance. These first and second coils 5 and 6 are connected through holes 9 and 9′ (please refer to FIG. 1F), for example, from both sides.
The fabrication procedure of the conventional magnetic field sensing device is largely divided into four steps: (1) preparing a substrate 1; (2) forming a first coil 5; (3) forming a magnetic substance 2; and (4) forming a second coil 6. More details on each step will now be provided.
First, as shown in FIG. 1A, a photoresist (layer) is applied to the prepared substrate 1 by a predetermined thickness, and first plating frames 7 have a plurality of grooves 7a formed through an exposing and a developing process. Then, by using an electroplating method, the grooves 7a of the plating frames 7 are filled with metals to form wires, and the plating frames 7 are removed. Here, before coating the substrate 1 with the photoresist, a seed film for plating is formed on the upper surface of the substrate 1. As a result, as shown in FIG. 1B, the first coil 5 is formed on the substrate 1.
Next, as shown in FIG. 1C, the first insulating film 3 is formed on the substrate 1 having the first coil 5 formed thereon. And, a soft magnetic film is layered on the first insulating film 3, and the magnetic substance 2 is formed, as shown in FIG. 1D, through the patterning and etching processes.
Later, the second insulating film 4 is formed on the magnetic substance 2 (please refer to FIG. 1E), and second plating frames 8 for forming the second coil 6 are formed on the second insulating film 4 as shown in FIGS. 1F and 1G. In detail, the through-holes 9 and 9′ communicate with both ends of the first coil and are formed on the second insulating film 4, and a seed layer (not shown) is formed thereon. Then, a photoresist (layer) is applied, and the second plating frames 8 are formed on the second insulating film 4, as shown in FIG. 1F, through the exposing and developing processes.
Once the second plating frames 8 are formed, metals are injected to the grooves 8a of the second plating frames 8 using the electroplating process, thereby forming a plurality of wires. Finally, the plating frames 8 are removed and the second coil 6 is formed.
A thusly fabricated magnetic field sensing device is illustrated in FIG. 1H.
Among others, the thickness of a layer formed during each fabrication process helps determine the small size of the magnetic field sensing device and the difficulty of the fabrication process. Also, it has a great impact on the degree of planarization of a substrate in every process, difficulty of the fabrication process, defective proportion, and performance of the magnetic field sensing device. However, in case of the conventional magnetic field sensing device, since the first coil 5 is protruded over the substrate 1, the insulating films 3 and 4 for use in insulation and planarization get thicker. This consequently puts impediments in the way of fabricating small-sized magnetic field sensing devices. Especially, since the magnetic substance 2 has a fixed thickness, a stepped portion should occur on the second insulating film 4. To prevent the stepped portion, manufacturers make the insulating films 3 and 4 even thicker.
However, if the thickness of the insulating films 3 and 4 is increased, not only the entire thickness of the device is also increased, but also the process (i.e., etching) for forming the through holes 9 and 9′ connecting the first coil 5 and the second coil 6 becomes very difficult, often resulting in a high inferiority rate.
Also, no matter how thick the second insulating film 4 may be coated, the upper surface of the second insulating film 4 cannot be planarized perfectly. The stepped portion causes a non-uniform distribution of exposure over the photoresist being deposited on the upper surface of the second insulating film 4. Ultimately, this increases the proportion of the second coil 6 that is defective.